Bridges are normally made using beams, which span a region to be covered, which are supported on abutments, and which have a flat deck spanning on top of the beams. The deck is almost always made of concrete that is poured in place into temporary formwork. While the beams have some problems, the deck is subject to many problems. These can be summarized in two main areas--the cost and difficulty of the forming and long term deterioration.
For example, bridges have been constructed using multiple parallel steel beams. However these beams suffer from corrosion induced by atmospheric pollutants, road salt, vehicle emissions, rain and bird excrement. Steel by its nature is very subject to corrosion. The ledge design of steel beams harbours dirt and pollutants that accelerate corrosion.
In order to avoid the problems of steel beams, precast prestressed concrete beams have been used. They are often referred to in the trade as "AASHTO" girders. Their configuration has a ledge design which inherently in the casting process leads to surface imperfections. The ledge also harbours dirt, pollutants, birds etc. which enter through the imperfections causing deterioration of the prestressing steel.
Both of the above described bridges are constructed with an ordinary poured in place concrete flat slab on top of the beams serving as the top deck.
Ordinary concrete decks suffer from severe long term deterioration. The deterioration is caused by water transmitted into the deck through the numerous pores and hairline cracks that are normal to an ordinary concrete deck. These pollutants reach the steel reinforcement causing it to rust and expand, which in turn causes the concrete to delaminate and eventually leads to collapse of the deck. Maintenance and repairs of concrete decks with rusted steel is difficult and costly.
The cracks in the concrete are present when the forces on the concrete are in tension and not compression. It is normal for there to be tension forces in a conventional concrete deck spanning across the tops of beams.
Prestressing concrete on the other hand is a method which compresses the concrete at very high pressures. This compresses the fine cracks and dramatically reduces the penetration of water and pollutants. To date beams have been prestressed or post tensioned, but the flat decks are not stressed and therefore are not under compression.
The construction of flat decks over open beams is difficult and costly. The formwork for these slabs is custom built, used only once and then removed, all by expensive skilled labour. The work is difficult due to the beams being very narrow, the beams being spaced many feet apart with the inherent danger to both the workers and traffic below. The formwork must be suspended between the beams to allow the deck to sit directly on the beams. The intricate dimensions mean that all formwork material is wasted and cannot be re-used. The labour to do this work is inefficient and very costly. If the bridges are in a rural area skilled labour have to be paid while travelling to the area.
The problem is worse at the outer edges of the bridge. The edge of the concrete deck is usually cantilevered and formed in complex shapes to receive guard rails, light posts etc. This edge condition is very labour intensive and costly. To avoid this costly labour as much as possible most bridges are usually utilitarian in design with very little architectural merit.
Another type of bridge is the poured in place solid concrete slab or beam. While these bridges appear simple, they are very difficult to construct because of the extensive scaffolding and formwork necessary to receive the poured in place concrete. This scaffolding and forming requires large crews of highly skilled workers, is very expensive and is very slow. These problems are compounded if traffic must continue on the road being spanned and therefore regular scaffolding cannot be used. This is normal if a bridge is being reconstructed or is located in an urban area. The disruption and cost to the community can be substantial.
Poured in place concrete bridges suffer from being very heavy and this limits their economical span. This weight can be reduced by forming voids inside or on the underside but this adds to the complexity, cost and time of construction. When voids are located inside the beam, they suffer from problems of water entering through cracks and accumulating inside the voids. The inside voids are also impossible to inspect.
Another type of bridge is the hollow box beam. This can either be cast in place or precast in pieces and installed segmentally with post-tensioning holding the pieces together in mid-air. While the poured in place hollow beams are more efficient than the solid beam with voids, the complexities and problems during construction are even greater. Segmental precast box beams are so expensive that they are only used for unusually large spans such as over wide bodies of water.